Nanodiamond (ND) particles have attracted a wide interest in the scientific community, and are still investigated, because they inherit most of the superior properties of bulk diamond (hardness and Young’s modulus, biocompatibility, non toxicity, high thermal conductivity and electrical resistivity, chemical inertness, environmental stability and resistance to harsh environments). They are excellent candidates for many applications in the fields of: drug delivery carriers, bio-labelling probes, nanocomposites, single photon sources, electrochemical energy storage, nucleation sites for growth of high performance CVD nanocrystalline diamond films, highly efficient and stable photocathodes , just to cite a few. Usually, the nanoparticles exhibit a very high surface-to-volume ratio and therefore they got surface-dependent properties more significantly than their bulk counterparts and tunable with treatments. For the above applications, the ND particles are generally handled in colloidal dispersion and their behavior in solution is critical. In particular, as-received particles with size of few nanometers are hardly separate in monodisperse particle colloids, but high temperature treatment in H2 gas or air makes easy to get monodispersion and stability.
In this contribution, two types of ND particles, with a similar average grain size of 250 nm, were investigated. They have different sp3 (diamond phase) and sp2 (graphite phase) carbon contents and for this they were named diamond-rich (D-R) and graphite-rich (G-R) NDs. The surface of ND particles was modified with treatment in H2 microwave plasma.
Many aqueous dispersions were prepared with the two types of powders, as received (NDas-rec) and hydrogenated (H-ND). The more concentrated (1 mg/1 ml) were used to produce photocathodes whose ND layers were deposited by pulsed spray technique , whereas more diluted (0.1 mg/1 ml) were examined by dynamic light scattering in the pH range from 2 to 12. The ND particles were also characterized by Raman and FTIR spectroscopies, and transmission electron microscopy.
The effect of the hydrogen treatment is beneficial increasing in the solid state the quantum efficiency of ND-based photocathodes  and in solution the particle zeta potential and therefore their stability.
 L. Velardi, A. Valentini, G. Cicala, UV photocathodes based on nanodiamond particles: effect of carbon hybridization on the efficiency, Diam. Relat. Mater. 76 (2017) 1–8.